Several temperature-sensitive cell cycle mutants of Saccharomyces cerevisiae have been found to accumulate chitin at the restrictive temperature. Extracts from a proteinase B negative mutant, derived from the same organism, catalyze the activation of chitin synthetase zymogen. An endochitinase was purified from yeast and partially characterized. The enzyme appears to be preferentially localized in the vacuole fraction. An activator of beta(1 yields 3) glucan synthetase found in boiled yeast extracts was purified and identified as GTP. A family of nucleotides containing two bases and three phosphates were found to be potent inhibitors of glucan synthetase.